FIG. 2 is a block diagram showing a conventional switching control circuit that includes a brownout detection function and a conventional AC/DC converter that employs the conventional switching control circuit. In FIG. 2, the conventional AC/DC converter includes high-frequency-cut filter 2 connected to AC power supply 1. The voltage of AC power supply 1 is Vin and the frequency thereof is 50 Hz, for example. (The high-frequency-cut filter 2 will be referred to simply as “noise filter 2” hereafter.) Noise filter 2 includes inductor 3, having an inductance L1, connected to AC power supply lines, and capacitor 4, connected between the AC power supply lines. An AC/DC converter of the foregoing description is disclosed, for example, in Unexamined Laid Open Japanese Patent Application Publication number Hie. 7 (1995)-131984 (FIGS. 1, 3, 5, and 7).
Full-wave rectifying circuit 5 includes rectifying diodes 6 and is connected to the output stage of noise filter 2. First smoothing capacitor 7, having a capacitance C1, is connected between the output terminal of full-wave rectifying circuit 5 and ground. Dividing resistor 9, having a resistance R1, and dividing resistor 10, having a resistance R2, are connected between output terminal 8, at which the voltage is Vm, and ground. The divided voltage of the voltage Vm, obtained from a connection point 11 of resistors 9 and 10, is connected to the negative input terminal of hysteresis comparator 22 in switching control circuit 20.
Conventional switching control circuit 20 includes a brownout detection function. Hysteresis comparator 22 compares the divided voltage, obtained by dividing the voltage Vm at output terminal 8 and connected to the negative terminal thereof, with a reference voltage 21 supplied to the positive input terminal thereof. When hysteresis comparator 22 detects that the divided voltage is lower than reference voltage 21, the output 23 of hysteresis comparator 22 feeds a high level signal (hereinafter referred to as an “H-level”) to timer 24.
Timer 24 counts up the time period for which the output from hysteresis comparator 22 is at the H-level. If the counted-up time period exceeds a predetermined time period, the output 25 of timer 24 feeds an H-level signal to one of the input terminals of NOR circuit 26. If output 23 from hysteresis comparator 22 returns to a low level (hereinafter referred to as an “L-level”) before the predetermined time period elapses, timer 24 is reset.
The other input terminal of NOR circuit 26 receives a high-frequency pulse signal 27 from a PWM control circuit (not shown) or from a PFM control circuit (not shown). Output 28 from NOR circuit 26 is fed to the gate terminal of switching device 15 and controls the ON and OFF state of switching device 15, which includes an N-channel MOSFET that is OFF when output 25 is at the H-level.
In the normal operation mode (excluding the operations for brownout detection), output 25 from timer 24 is set at the L-level. The L-level is fed as one of the inputs to NOR circuit 26, and high-frequency pulse signal 27 is fed as the other one of the inputs to NOR circuit 26. Therefore, NOR circuit 26 lets high-frequency pulse signal 27 pass without modification as output 28. Since output 28 is applied to the gate terminal of switching device 15, the switching ON and OFF of switching device 15 is controlled by output 28.
Output terminal 8 of full-wave rectifying circuit 5 is connected in series to reverse blocking diode 13 via inductor 12, having an inductance L2, as an energy storage element. Switching device 15 and resistor 16 are connected in series with each other between a connection point 14 of inductor 12 to reverse blocking diode 13 and ground. A second smoothing capacitor 18, having a capacitance C2, is connected between the ground and cathode terminal 17 of reverse blocking diode 13, the voltage at that point being denoted by VO. The AC/DC converter is configured such that a desired DC voltage is obtained as the cathode terminal voltage VO.
The brownout detection function will now be described. Generally, “brownout” implies that the power supply voltage is lower than a specified lower limit but higher than 0 V temporarily, and the apparatus is in an intermediate state immediately before stopping functioning (blackout). In the intermediate state, the apparatus cannot keep the normal state any more. Usually, the apparatuses are designed to perform a brownout detection function before the apparatus stops functioning (before a blackout occurs).
In a conventional switching control circuit 20, as shown in FIG. 2, that includes a brownout detection function, timer 24 starts operating if it is detected that the voltage Vm at output terminal 8 of full-wave rectifying circuit 5 drops below brownout (stopping) voltage level 29 as shown in the timing chart in FIG. 3. If the state in which the voltage Vm is below brownout voltage level 29 continues, timer 24 feeds output 25 set at the H-level to one of the input terminals of NOR circuit 26 after a predetermined time period set in timer 24 elapses. H-level output 25 from timer 24 sets output 28 from NOR circuit 26 at the L-level to control switching device 15 such that switching device 15 stops switching.
In the following, the input voltage at the instance at which switching device 15 stops switching is denoted by Vin0. The switching frequency (that is the frequency of pulse signal 27 inputted to NOR circuit 26) is significantly higher than the frequency of the voltage Vin (usually 50 to 60 Hz) inputted to noise filter 2 from AC power supply 1. Therefore, in an operation analysis based on the time width (on the switching period level) for which the current of inductor 12 varies, it may be reasonably assumed that the input voltage Vin0 at the instance at which switching device 15 stops switching is maintained. Described from another viewpoint, it may be reasonably assumed that the output voltage (cathode terminal voltage) VO does not vary with time (in the transient response at the instant at which the switching stops).
If the assumptions as described above are held, it may be reasonably expected that the voltage Vm at output terminal 8 of full-wave rectifying circuit 5 does not vary significantly. Therefore, the current that flows through inductor 3 increases when the input voltage value Vin is high enough to exceed Vm. When the input voltage Vin is low, the current that flows through inductor 3 decreases. In the steady state, the average value of the current that flows through inductor 3 is equal to the average value of the current that flows through inductor 12. Since the resistance values R1 and R2 of resistors 9 and 10 connected between output terminal 8 of full-wave rectifying circuit 5 and the ground are large, the current that flows through resistors 9 and 10 can be ignored.
If a brownout is detected and the switching is stopped (switching device 15 is made to be OFF), the current that flows through inductor 12, is reduced at a rate of (VO−Vm)/L2. However, the current that flows through inductor 12 keeps charging second smoothing capacitor 18 until the current value becomes 0. The current to inductor 12 is fed from first smoothing capacitor 7 and inductor 3. When the input voltage Vin0 at the instant at which switching is stopped is higher than Vm, the current of inductor 12 keeps decreasing but the current of inductor 3 keeps increasing. The current of inductor 3 keeps increasing until the input voltage Vin0 becomes equal to or lower than Vm. (If Vin0−Vm is expressed by ΔV, the current of inductor 3 changes at the rate of ΔV/L1, when ΔV is larger than 0. The current of inductor 3 increases when ΔV is higher than 0, but decreases when ΔV is lower than 0.)
When the current that flows through inductor 12 immediately after the switching stops is higher than the current that flows through inductor 3, the voltage Vm becomes lower. As the voltage Vm becomes lower, the decreasing rate of the current that flows through inductor 12 and the increasing rate of the current that flows through inductor 3 become larger and larger. Finally, the magnitude relation between the current that flows through inductor 12 and the current that flows through inductor 3 is reversed.
When the current that flows through inductor 12 is lower than the current that flows through inductor 3, Vm increases. The current that flows through inductor 3 keeps increasing until Vm becomes higher than Vin0. Since the circuit that charges second smoothing capacitor 18 via inductor 12 constitutes a booster circuit as shown in FIG. 2, the relational expression VO>Vm, Vin0 holds. Due to the relations described above, the current that flows through inductor 12 keeps decreasing and reaches 0 finally.
Even when Vin0 is equal to or lower than Vm, the current that flows through inductor 12 will become 0 before the current that flows through inductor 3 becomes 0, if Vm is high to an extent such that the absolute value of ΔV=Vin0−Vm is small and the current that flows through inductor 3 is high.
In the state in which the current that flows through inductor 12 is 0 and a current is still flowing through inductor 3, the right end of inductor 12 is brought into an open state and the current flowing through inductor 3 flows entirely into first smoothing capacitor 7. In other words, a resonance operation based on L1 that is, the inductance value of inductor 3) and C1 (that is, the capacitance value of smoothing capacitor 7) starts and Vm rises to the peak of the resonance. The peak of the resonance is higher as the value of the current that flows through inductor 3 at the start of the resonance is higher.
However, inductance 3 in noise filter 2 in the front stage of full-wave rectifying circuit 5 makes a current flow even when the switching is stopping. Therefore, the input voltage Vm for brownout detection will rise as shown in FIGS. 4(a) and 4(b), if the switching stops in the range in which the phase angle is high and a high current is flowing. If the rising input voltage Vm exceeds brown-in (recovery) voltage level 30 shown in FIG. 3, the switching operation will recover from the stopped state thereof caused by a brownout (an improper brown-in operation will be caused) before the conditions for the recovery from the stopping state caused by a brownout are obtained. The improper brown-in operation de-stabilizes the switching operation of the AC/DC converter.
In view of the foregoing, it would be desirable to provide a switching control circuit that stops the switching of a switching device in a low phase-angle range of an AC power supply, in which a not-so-high current is flowing, to prevent an improper recovery operation from occurring.